Magnetic memory circuits are based on magneto-resistive behavior of magnetic storage elements that are integrated typically with a complementary metal-oxide-semiconductor (CMOS) technology. Such memory circuits generally provide non-volatility and an unlimited read and write capability. An example is the magnetic random access memory (MRAM) circuit that includes a plurality of bit cells, each defining an addressable magnetic storage element that may include a magnetic tunnel junction (MTJ) stack.
Each addressable MTJ stack can have a free layer whose magnetic spin orientation may be flipped between two states by the application of a magnetic field induced by energizing write conductors.
FIG. 1 illustrates a partly schematic and partly cross-sectional view of a commonly used magnetic bit cell 100. The bit cell 100 is generally usable in an array in magnetic memory circuits, such as magnetic random access memory (MRAM) circuits. An MTJ stack 120 is shown to form an addressable magnetic storage element in the bit cell 100. The MTJ stack 120 is generally designed to be integrated into a back-end metallization structure following a front-end CMOS processing. The MTJ stack 120 is shown to be provided between a first metallization layer Mx and a second metallization layer My, wherein the MTJ stack 120 is connected to the first layer Mx through a via hole 116 and to the second layer My through a via hole 104. The MTJ stack 120 includes a fixed layer 110, a tunnel oxide layer 108, a free layer 106 and an extended bottom electrode 112. The second layer My is patterned to include a bit line (BL) 102 corresponding to the MTJ stack 120. An address transistor 122 is also shown schematically that is connected to the first layer Mx by a connection 118a. The address transistor 122 is usable for reading the state of the MTJ stack 120. The first layer Mx is also patterned to include a read word line (WL) 118b for selectively operating the address transistor 122 and to include a write word line (WL) 118c for writing into the MTJ stack 120. The write WL 118c has no contact with the bottom electrode 112, and when energized, induces a magnetic field at a junction in the MTJ stack 120.